linux-kernel - Re: [PATCH] extend e2fsprogs functionality to add EXT2_FLAG

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Message-ID: <4B4CA868.2080204@redhat.com>
Date:	Tue, 12 Jan 2010 11:50:48 -0500
From:	Ric Wheeler <rwheeler@...hat.com>
To:	Michal Novotny <minovotn@...hat.com>
CC:	Christoph Hellwig <hch@...radead.org>, linux-ext4@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] extend e2fsprogs functionality to add EXT2_FLAG_DIRECT
 option

On 01/12/2010 11:43 AM, Michal Novotny wrote:
> On 01/12/2010 05:38 PM, Christoph Hellwig wrote:
>> Ok, I looked at the issue. The problem is that the Xen backend drivers
>> are (as expected) utterly braindead and submit bios directly from the
>> virtualization backed without using proper abstractions and thus
>> bypassing all the cache coherency features in the fileystems (the block
>> device nodes are just another mini-filesystem in that respect). So
>> when you first have buffered access in the host pages may stay in cache
>> and get overwritten directly on disk by a Xen guest, and once the guest
>> is down the host may still use the now stale cached data.
>>
>> I would recommend to migrate your cutomers to KVM which uses the proper
>> abtractions and thus doesn't have this problem. There's a reason after
>> all why all the Xen dom0 mess never got merged to mainline.
> So, do you think the problem is in the Xen backend drivers and to make
> it working right in Xen the driver fix is needed?

If XEN drivers by pass the normal IO and FS stack on the host, then I can 
understand why the hack to e2fsprogs works but it does not seem like a good fix.

Specifically, the data will continue to be cached (and if dirty, might be 
written back to the storage eventually).

If we need a work around, you need to drop VM caches for that device before you 
update the guest's files and possibly again afterwards (and make sure that 
nothing pulls the data into cache during the operation).

Basically, this sounds like the backend drivers are doing something really, 
really dangerous....

ric

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